Memory has a major role in the computer system, it also depends on the type of memory.

RAM- Random access Memory
It deals with the cache memory or to be precise, RAM memory is used to connect the processor with the hard disk, cd-rom, printer, display screen. When the computer boots at the initial stage, the processor first looks for the RAM memory and with the help of that it loads the connecting drivers between the hardware equipment (harddisk,screen,printer) and the processor and after compiling the same we gets the display and the Graphical User Interface option along with the keyboard and all the external device which are connected.

Other than this, it is only the RAM memory which is used while we are running any application, in case if our RAM memory is fully used lets say 1GB RAM is fully used then you would not be able to launch one more application. It would hang the system.
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